[57.17] Spectropolarimetry of Broad Absorption Line Quasars

P. M. Ogle (Caltech)

We have made detailed spectropolarimetric observations of sixteen
broad absorption line quasars (BALQSO) with the W.M. Keck Telescope.
Most of these objects show polarized continuum emission with polarization
P<2% and unpolarized broad emission lines (BEL). The polarization rises
dramatically in the broad absorption troughs to values as large as 12%.
We conclude that scattering by electrons or dust within the BEL region (BELR)
is the most likely source of polarization, and put limits on the size of
the scattering region relative to the BELR. Next, we use a scattering plus
obscuration model to describe the continuum and trough polarizations. This
model allows us to put constraints on the disposition of the BAL region (BALR)
with respect to the scattering region as a function of radial velocity.
A calculation of the polarization distribution function for axisymmetric
scattering and random lines of sight does not match the observed distribution.
In particular, three of the BALQSO have continuum polarizations greater than
3%, and appear to be part of a separate population. The large range in
polarization observed for the remaining objects (P=0.1-1.8%) argues for
either a polar BALR flow or a large range in scattering efficiency. In most
of the BALQSO there is no convincing evidence of polarized resonantly
scattered light, though it must surely be present at some level.